1. Field of the Invention
This invention relates to devices used in drilling and boring through subterranean formations. More particularly, this invention relates to a polycrystalline diamond or other superabrasive cutter intended to be installed on a drill bit or other tool used for earth or rock boring, such as may occur in the drilling or enlarging of an oil, gas, geothermal or other subterranean borehole, and to bits and tools so equipped.
2. State of the Art
There are three types of bits which are generally used to drill through subterranean formations. These bit types are: (a) percussion bits (also called impact bits); (b) rolling cone bits, including, tri-cone bits; and (c) drag bits or fixed cutter rotary bits (including core bits so configured), the majority of which currently employ diamond or other superabrasive cutters, polycrystalline diamond compact (PDC) cutters being most prevalent.
In addition, there are other structures employed downhole, generically termed "tools" herein, which are employed to cut or enlarge a borehole or which may employ superabrasive cutters, inserts or plugs on the surface thereof as cutters or wear-prevention elements. Such tools might include, merely by way of example, reamers, stabilizers, tool joints, wear knots and steering tools. There are also formation cutting tools employed in subterranean mining, such as drills and boring tools.
Percussion bits are used with boring apparatus known in the art that moves through a geologic formation by a series of successive impacts against the formation, causing a breaking and loosening of the material of the formation. It is expected that the cutter of the invention will have use in the field of percussion bits.
Bits referred to in the art as rock bits, tri-cone bits or rolling cone bits (hereinafter "rolling cone bits") are used to bore through a variety of geologic formations, and demonstrate high efficiency in firmer rock types. Prior art rolling cone bits tend to be somewhat less expensive than PDC drag bits, with limited performance in comparison. However, they have good durability in many hard-to-drill formations. An exemplary prior art rolling cone bit is shown in FIG. 2. A typical rolling cone bit operates by the use of three rotatable cones oriented substantially transversely to the bit axis in a triangular arrangement, with the narrow cone ends facing a point in the center of the triangle which they form. The cones have cutters formed or placed on their surfaces. Rolling of the cones in use due to rotation of the bit about its axis causes the cutters to embed into hard rock formations and remove formation material by a crushing action. Prior art rolling cone bits may achieve a rate of penetration (ROP) through a hard rock formation ranging from less than one foot per hour up to about thirty feet per hour. It is expected that the cutter of the invention will have use in the field of rolling cone bits as a cone insert for a rolling cone, as a gage cutter or trimmer, and on wear pads on the gage.
A third type of bit used in the prior art is a drag bit or fixed-cutter bit. An exemplary drag bit is shown in FIG. 1. The drag bit of FIG. 1 is designed to be turned in a clockwise direction (looking downward at a bit being used in a hole, or counterclockwise if looking at the bit from its cutting end as shown in FIG. 1) about its longitudinal axis. The majority of current drag bit designs employ diamond cutters comprising polycrystalline diamond compacts (PDCs) mounted to a substrate, typically of cemented tungsten carbide (WC). State-of-the-art drag bits may achieve an ROP ranging from about one to in excess of one thousand feet per hour. A disadvantage of state-of-the-art PDC drag bits is that they may prematurely wear due to impact failure of the PDC cutters, as such cutters may be damaged very quickly if used in highly stressed or tougher formations composed of limestones, dolomites, anhydrites, cemented sandstones, interbedded formations such as shale with sequences of sandstone, limestone and dolomites, or formations containing hard "stringers." It is expected that the cutter of the invention will have use in the field of drag bits as a cutter, as a gage cutter or trimmer, and on wear pads on the gage.
As noted above, there are additional categories of structures or "tools" employed in boreholes, which tools employ superabrasive elements for cutting or wear prevention purposes, including reamers, stabilizers, tool joints, wear knots and steering tools. It is expected that the cutter of the present invention will have use in the field of such downhole tools for such purposes, as well as in drilling and boring tools employed in subterranean mining.
It has been known in the art for many years that PDC cutters perform well on drag bits. A PDC cutter typically has a diamond layer or table formed under high temperature and pressure conditions to a cemented carbide substrate (such as cemented tungsten carbide) containing a metal binder or catalyst such as cobalt. The substrate may be brazed or otherwise joined to an attachment member such as a stud or to a cylindrical backing element to enhance its affixation to the bit face. The cutting element may be mounted to a drill bit either by press-fitting or otherwise locking the stud into a receptacle on a steel-body drag bit, or by brazing the cutter substrate (with or without cylindrical backing) directly into a preformed pocket, socket or other receptacle on the face of a bit body, as on a matrix-type bit formed of WC particles cast in a solidified, usually copper-based, binder as known in the art.
A PDC is normally fabricated by placing a disk-shaped, cemented carbide substrate into a container or cartridge with a layer of diamond crystals or grains loaded into the cartridge adjacent one face of the substrate. A number of such cartridges are typically loaded into an ultra-high pressure press. The substrates and adjacent diamond crystal layers are then compressed under ultra-high temperature and pressure conditions. The ultra-high pressure and temperature conditions cause the metal binder from the substrate body to become liquid and sweep from the region behind the substrate face next to the diamond layer through the diamond grains and act as a reactive liquid phase to promote a sintering of the diamond grains to form the polycrystalline diamond structure. As a result, the diamond grains become mutually bonded to form a diamond table over the substrate face, which diamond table is also bonded to the substrate face. The metal binder may remain in the diamond layer within the pores existing between the diamond grains or may be removed and optionally replaced by another material, as known in the art, to form a so-called thermally stable diamond ("TSD"). The binder is removed by leaching or the diamond table is formed with silicon, a material having a coefficient of thermal expansion (CTE) similar to that of diamond. Variations of this general process exist in the art, but this detail is provided so that the reader will understand the concept of sintering a diamond layer onto a substrate in order to form a PDC cutter. For more background information concerning processes used to form polycrystalline diamond cutters, the reader is directed to U.S. Pat. No. 3,745,623, issued on Jul. 17, 1973, in the name of Wentorf, Jr. et al.
Prior art PDCs experience durability problems in high load applications. They have an undesirable tendency to crack, spill and break when exposed to hard, tough or highly stressed geologic structures so that the cutters sustain high loads and impact forces. They are similarly weak when placed under high loads from a variety of angles. The durability problems of prior art PDCs are worsened by the dynamic nature of both normal and torsional loading during the drilling process, wherein the bit face moves into and out of contact with the uncut formation material forming the bottom of the wellbore, the loading being further aggravated in some bit designs and in some formations by so-called bit "whirl."
The diamond table/substrate interface of conventional PDCs is subject to high residual stresses arising from formation of the cutting element, as during cooling, the differing coefficients of thermal expansion of the diamond and substrate material result in thermally-induced stresses. In addition, finite element analysis (FEA) has demonstrated that high tensile stresses exist in a localized region in the outer cylindrical substrate surface and internally in the substrate. Both of these phenomena are deleterious to the life of the cutting element during drilling operations as the stresses, when augmented by stresses attributable to the loading of the cutting element by the formation, may cause spalling, fracture or even delamination of the diamond table from the substrate.
Further, high tangential loading of the cutting edge of the cutting element results in bending stresses on the diamond table, which is relatively weak in tension and will thus fracture easily if not adequately supported against bending. The metal carbide substrate on which the diamond table is formed is are typically of inadequate stiffness to provide a desirable degree of such support.
The relatively thin diamond table of a conventional PDC cutter, in combination with the substrate, also provide lower than optimum heat transfer from the cutting edge of the cutting face, and external cooling of the diamond table as by directed drilling fluid flow from nozzles on the bit face is only partially effective in reducing the potential for heat-induced damage.
The relatively rapid wear of conventional, thin diamond tables of PDC cutters also results in rapid formation of a wear flat in the substrate backing the cutting edge, the wear flat reducing the per-unit area loading in the vicinity of the cutting edge and requiring greater weight on bit (WOB) to maintain rate of penetration (ROP). The wear flat, due to the introduction of the substrate material as a contact surface with the formation, also increases drag or frictional contact between the cutter and the formation due to modification of the coefficient of friction. As one result, frictional heat generation is increased, elevating temperatures in the cutter, while at the same time, the presence of the wear flat reduces the opportunity for access by drilling fluid to the immediate rear of the cutting edge of the diamond table.
Others have previously attempted to enhance the durability of conventional PDC cutters. By way of example, the reader is directed to U.S. Pat. No. 32,036 to Dennis (the '036 patent); U.S. Pat. No. 4,592,433 to Dennis (the '433 patent); and U.S. Pat. No. 5,120,327 to Dennis (the '327 patent). In FIG. 5A of the '036 patent, a cutter with a beveled peripheral edge is depicted, and briefly discussed at col. 3, lines 51-54. In FIG. 4 of the '433 patent, a very minor beveling of the peripheral edge of the cutter substrate or blank having grooves of diamond therein is shown (see col. 5, lines 1-2 of the patent for a brief discussion of the bevel). Similarly, in FIGS. 1-6 of the '327 patent, a minor peripheral bevel is shown (see col. 5, lines 40-42 for a brief discussion of the bevel). Such bevels or chamfers were originally designed to protect the cutting edge of the PDC while a stud carrying the cutting element was pressed into a pocket in the bit face. However, it was subsequently recognized that the bevel or chamfer protected the cutting edge from load-induced stress concentrations by providing a small load-bearing area which lowers unit stress during the initial stages of drilling. The cutter loading may otherwise cause chipping or spalling of the diamond layer at an unchamfered cutting edge shortly after a cutter is put into service and before the cutter naturally abrades to a flat surface or "wear flat" at the cutting edge.
It is also known in the art to radius, rather than chamfer, a cutting edge of a PDC cutter, as disclosed in U.S. Pat. No. 5,016,718 to Tandberg. Such radiusing has been demonstrated to provide a load-bearing area similar to that of a small peripheral chamfer on the cutting face.
U.S. Pat. No. 5,351,772 to Smith discloses a PDC cutter having a plurality of internal radial lands to interrupt and redistribute the stress fields at and adjacent the diamond table/substrate interface and provide additional surface area for diamond table/substrate bonding, permitting and promoting the use of a thicker diamond table useful for cutting highly abrasive formations.
U.S. Pat. No. 5,435,403 to Tibbitts discloses a PDC cutter employing a bar-type, laterally-extending stiffening structure adjacent the diamond table to reinforce the table against bending stresses.
For other approaches to enhance cutter wear and durability characteristics, the reader is also referred to U.S. Pat. No. 5,437,343, issued on Aug. 1, 1995, in the name of Cooley et al. (the '343 patent); and U.S. Pat. No. 5,460,233, issued on Oct. 24, 1995, in the name of Meany et al. (the '233 patent). In FIGS. 3 and 5 of the '343 patent, it can be seen that multiple, adjacent chamfers are formed at the periphery of the diamond layer (see col. 4, lines 31-68 and cols. 5-6 in their entirety). In FIG. 2 of the '233 patent, it can be seen that the tungsten carbide substrate backing the superabrasive table is tapered at about 10-15.degree. to its longitudinal axis to provide some additional support against catastrophic failure of the diamond layer (see col. 5, lines 2-67 and col. 6, lines 1-21 of the '233 patent). See also U.S. Pat. No. 5,443,565, issued on Aug. 22, 1995, in the name of Strange for another disclosure of a multi-chamfered diamond table.
While the foregoing patents have achieved some enhancement of cutter durability, there remains a great deal of room for improvement, particularly when it is desired to fabricate a cutter having, as desirable features, a relatively larger and robust diamond volume offering reduced cutter wear characteristics and increased stiffness. Conventional PDCs employ a diamond table on the order of about 0.030 inch thickness. So-called "double-thick", or 0.060 inch thick, diamond tables have been attempted, but without great success due to low strength and wear resistance precipitated to some degree by poorly-sintered diamond tables. It has even been proposed to fabricate PDC cutters with still-thicker chamfered diamond tables, as thick as 0.118 inch, as disclosed in U.S. Pat. No. 4,792,001 to Zijsling. However, the inventors are not aware of the actual manufacture of any such cutters.